So - I've had repetitive stress injuries on and off for years. I'll tell you the one thing that really has worked for me to deal with them: Alexander Technique.
I'm not sure if your injury is a one-time deal. If it is, probably just time away from the instrument to rest, and coming back in with a more relaxed approach (probably a less strenuous piece) is ok. But if the pain or weakness is chronic, you really need to reinvent your approach. Alexander Technique essentially teaches you to re-map your neuromuscular system to achieve this.
Hope that provides a ray of hope. It's made a big difference for me.